It is proposed to study specific biochemical steps in the transformation of cholesterol to chenodeoxycholic acid in experimental animals (rat, guinea pig, rabbit), in vivo, in the isolated perfused liver and in subcellular fractions of liver homogenates. Emphasis will be placed on the mechanism of degradation of the cholesterol side chain, i.e., the sites and sequences of various hydroxylation reactions and loss of the terminal isopropyl group. It is further proposed to study the biological properties of the naturally occurring C27bile alcohols hydroxylated at C-25 and of their synthetic 24-nor-analogs, hydroxylated at C-25. Emphasis will be placed on their intestinal absorption, their effect on those intestinal bacteria which dehydrogenate and dehydroxylate bile acids, on cholesterol balance and on the rate-limiting enzymes of cholesterol and bile acid biosynthesis.